This invention relates to a carrier for a developer for developing an electrostatic latent image, to an electrostatic latent image developer using the carrier, and to an image forming method by electrophotography, electrostatic recording, electrostatic printing, etc. using the developer.
In the field of image forming apparatuses utilizing electrophotography, such as copying machines and printers, various studies are now made on an electrostatic latent image developing system for the purpose of improving the duration and images reproducibility. In such an image forming apparatus, an electrostatic latent image formed on an image forming member such as a photoconductor is developed with a developer supported on a developer carrying member. One-component type developers composed of a magnetic or non-magnetic toner and two-component type developers composed of a toner and a magnetic carrier are known as the developer. The two-component type developers are advantageous over the one-component type developer, because of easiness in high speed toner feeding and in uniformizing chargeability of toner, which characteristics in turn permit high speed image forming and production of high grade images.
In development of an electrostatic latent image on an image forming member with a two-component type developer supported on a developer carrying member, an electric field is formed in a gap between the latent image bearing member serving as an electrode and the developer carrying member serving as a counter electrode, in which gap the carrier is present. Thus, since the carrier present between the electrodes has an influence upon the electric field, the electrical properties of the carrier has a great influence upon the quality of image produced. It is therefore important that the carrier should have uniform electrical properties in order to improve the image quality and for preventing image defects. In particular, it is important that a difference in electric resistance between carrier particles should be minimized in order to prevent deposition of the carrier onto surfaces of the image forming member such as a photoconductor (carrier deposition). Such carrier deposition is apt to occur when the electric resistance of the carrier particles is not uniform. Susceptibility of a carrier to dielectric breakdown is also a cause of carrier deposition. In addition, image defects such as white spots and discharge marks are also caused as a result of the dielectric breakdown of the carrier, especially when an AC bias having a large amplitude is applied between the image forming member and the developer carrying member.
Japanese Laid-Open Patent Publications No. H09-319,161, No. H09-269614 and H10-186731 disclose a carrier including core particles each surrounded by an outer layer containing a resin matrix in which thermosetting resin particles and fine particles of an electric conductivity imparting material are dispersed for improve anti-spent property (prevention of deposition of toner components onto carrier) and strength of the outer layer and for controlling electrical properties of the carrier. The known carrier, however, does not solve the problem of variation of electric resistance between carrier particles.
In the development of an electrostatic latent image with a two-component type developer using the carrier as a magnetic brush, it is proposed to displace the image forming member and the developer carrying member at different linear speeds for the purpose of ensure a sufficient amount of the developer which comes in contact with the latent image in a developing zone. However, such a difference in liner speeds brings about the following abnormal images: (a) a solid image has a portion in which the image density is low or almost zero (white) at an end portion thereof in a displacing direction of the latent-image-bearing image forming member, (b) a halftone image has a portion in which the image density is low or almost zero (white) at an end portion thereof in a displacing direction of the latent-image-bearing image forming member, and (c) the image density is changed at the boundary between the solid image and the halftone image. Such abnormal images are apt to appear at the boundary between the adjacent latent images in which the electric potentials of latent image abruptly change discontinuously. Such abnormal images are thus considered to result from the facts that the toner in the magnetic brush can move due to the sliding contact between the magnetic brush and the latent image and that a layer of developer, which is a dielectric member, passes through a discontinuous electric field.